Voice over Internet Protocol (VOIP) telephony calls can be routed in an IMS network using a telephone number mapping (ENUM) database. FIG. 1 is a flow diagram of routing a voice call in an IMS network using ENUM. For purposes of illustration and example, the call is between a caller at a home network (HN) 10 and a callee at another HN 12. From user equipment 14, a caller dials an E.164 number of a callee. At step 200, the E.164 number is passed from the user equipment 14 to a Proxy Call Session Control Function (P-CSCF) 16 of the caller's HN 10. Based on the E.164 number, the P-CSCF 16 sends an invite (Request URI) message and passes it to a Serving-CSCF (S-CSCF) 20 of the caller's HN 10, at step 202. Moving to step 204, the S-CSCF 20 queries an ENUM database 22 of the caller's HN 10 to attempt to resolve the E.164 number to ‘a Uniform Resource Indicator (URI).
Under desirable circumstances, at step 206, the ENUM database 22 will return a Session Initiation Protocol (SIP) URI or a telephone (TEL) URI to the S-CSCF 20 based on the query. Based on the SIP/TEL URI, at step 208, the S-CSCF 20 queries a Domain Name System (DNS) 24 of the caller's HN 10 to resolve a domain name of the URI to an Internet Protocol (IP) address. Proceeding to step 210, the DNS 24 returns the IP address of the URI domain to the S-CSCF 20. At step 212, the IP address is passed to an Interrogating-CSCF (I-CSCF) 26 of the callee's HN 12.
In a particular embodiment, using the IP address, the S-CSCF 20 of the caller's HN 10 sends a SIP INVITE request to the I-CSCF 26 of the callee's HN 12. At step 214, the I-CSCF 26 locates an S-CSCF 30 of the callee's HN 12, sends a SIP INVITE request to the S-CSCF 30. At step 116, the S-CSCF 30 passes the SIP invite request to a P-CSCF 32 of the callee's HN 12. At step 218, the P-CSCF 32 passes the SIP INVITE request to user equipment 34 of the callee.
In some circumstances, the ENUM database 22 may not return a SIP URI or a TEL URI in response to its being queried. In this case, the call is not routed using the IMS core but rather via a public switched telephone network (PSTN). Furthermore, if an entry for the E.164 number is not in the ENUM database, the caller is commonly routed to an announcement which indicates an error condition.